More of an annoying bug report than a hint, but I searched for some time before fixing the issue.

After updating to 10.4.8, my cursor suddenly had an annoying black rectangle around it. The rectangle can be turned off by first turning on the System Preferences -> Universal Access -> Seeing tab. In the Zoom function area, select Options, and un-select "Show preview rectangle when zoomed out."

Just in case it is an installer bug and not just my setup.

[robg adds: This is apparently a fairly-common occurence, as Apple saw fit to create this kbase article about it. You will only be able to see the preview rectangle if you've specified a "maximum zoom" in the sliders at the top of the options panel.]